Hydrotreating
The use of hydrotreating (HDT) to upgrade hydrocarbon fractions for use as charge stocks for catalytic cracking was well known in the art by the 1960's. Hydrotreating, as used herein, is meant to encompass those processes using hydrogen in the presence of catalysts in order to remove undesirable compounds from hydrocarbons, that is, to upgrade the hydrocarbons.
By 1960, it was recognized that hydrotreatment could be used for demetalation, desulfurization, Conradson Cabon Residue reduction and denitrogenation. There was universal recognition at that time in the art that hydrogenation catalysts comprising Group VI (Cr, Mo, W) and Group VIII metals or their oxides or sulfides deposited on porous refractory supports were extremely useful in hydrotreating processes. Preferred catalysts for hydrotreating were considered to be cobalt-molybdate or nickelmolybdate supported on alumina. These catalysts are generally referred to as "conventional HDT catalysts."
The pore size distribution of the HDT catalyst is a very important parameter in determination of the activity of the catalyst. Large pore catalysts generally possess greater demetalating activity and smaller pore catalysts generally possess lower demetalation activity, but higher desulfurization activity. U.S. Pat. No. 3,730,879 teaches an HDT process comprising a multi-layered arrangement of catalyst with different pore distributions. In the first bed, there is used a smaller pore catalyst which is more selective for desulfurization; and in the second downstream bed, there is used a larger pore catalyst which is more selective for removal of metal contaminants. According to U.S. Pat. No. 3,730,879, the desulfurization catalyst of the first bed has a catalyst characterized by a pore diameter distribution as follows: less than 25% 0-100 A; greater than 50% 100-200 A; and the remainder 200-600 A. The demetalation catalyst of the second bed has a catalyst characterized by a pore diameter distribution as follows: less than 20% 0-100 A; less than 45% 100-200 A; and balance 200-600 A.
The average pore diameter size for HDT catalysts in desulfurization processes is usually 100-200 A. Such average pore diameter size is disclosed in U.S. Pat. Nos. 3,393,148; 3,674,680; 3,764,565; 3,841,995 and 3,882,049.
Processes for the demetalation and desulfurization of oil fractions using conventional HDT catalysts with at least 60% of its pore volume in pores of 100 A to 200 A diameter and at least 5% of its pore volume in pores having diameters greater than 500 A are disclosed in U.S. Pat. Nos. 3,876,523 and 4,016,067.
U.S. Pat. No. 3,902,991 discloses a hydrodesulfurization process for oil fractions which uses a conventional HDT catalyst having at least 50% of its total pore volume in pores having a diameter size range of 65 to 150 A. Another hydrodesulfurization process for oil fractions is described in U.S. Pat. No. 3,730,879, wherein one of the catalysts has at least 50% of its total pore volume in pores having radii in the size range of 50-100 A. Still another hydrodesulfurization process is disclosed in U.S. Pat. No. 3,814,683. In this patent, the conventional HDT catalyst is characterized by having at least 65% of its total pore volume in pores having a diameter size of 80-180 A.
Other hydrodesulfurization processes using a conventional HDT catalyst having a specific pore size distribution are disclosed in U.S. Pat. Nos. 4,032,435; 4,051,021; 4,069,139 and 4,073,718.